High high above the water is the light and what color (K) is the bulb? What is the footprint of the tank, it is a 20g long or high? Only ask as there are so many different tanks out there now, what used to be a standard size for a tank is becoming hard to find.

But a 55w 50/50 10K/actinic will most likely do 'okay' at mid tank depths or higher, though 4-5 watts (or more) per gallon would be better.